Deep hypothermia and rewarming alters glutamate levels and glycogen content in cultured astrocytes

The Glutamate-Glutamine Cycle in Epilepsy

Epilepsy is a complex, multifactorial disease characterized by spontaneous recurrent seizures and an increased incidence of comorbid conditions such as anxiety, depression, cognitive dysfunction, and sudden unexpected death. About 70 million people worldwide are estimated to suffer from epilepsy, and up to one-third of all people with epilepsy are expected to be refractory to current medications. Development of more effective and specific antiepileptic interventions is therefore requisite. Perturbations in the brain's glutamate-glutamine cycle, such as increased extracellular levels of glutamate, loss of astroglial glutamine synthetase, and changes in glutaminase and glutamate dehydrogenase, are frequently encountered in patients with epilepsy. Hence, manipulations of discrete glutamate-glutamine cycle components may represent novel approaches to treat the disease. The goal of his review is to discuss some of the glutamate-glutamine cycle components that are altered in epilepsy, particularly neurotransmitters and metabolites, enzymes, amino acid transporters, and glutamate receptors. We will also review approaches that potentially could be used in humans to target the glutamate-glutamine cycle. Examples of such approaches are treatment with glutamate receptor blockers, glutamate scavenging, dietary intervention, and hypothermia.

The role of hypothermia in the regulation of blood glutamate levels in naive rats

BACKGROUND

The exact mechanism of hypothermia-induced neuroprotection has not been determined yet; however, we hypothesized that it may be mediated by a blood glutamate-scavenging effect. Here, we examine the effect of hypothermic conditions (mild, moderate, and deep) on blood glutamate levels in naive rats. To identify the mechanism of hypothermia-induced glutamate reduction, we also measured concentrations of glutamate oxaloacetate transaminase (GOT) and glutamate pyruvate transaminase (GPT), the primary regulators of glutamate concentration in blood.

METHODS

Rats were anesthetized with isoflurane, and their rectal temperature was maintained for 6 hours at 36 to 37°C, 33 to 36°C, 30 to 32°C, 18 to 22°C, or was not maintained artificially. At 6 hours, active cooling was discontinued and rats were allowed to rewarm. There were 12 rats in each group for a total of 60 rats. Blood samples were drawn at 0, 3, 6, 12, 24, and 48 hours for the determination of blood glutamate, GOT, and GPT levels.

RESULTS

A strong correlation between body temperature and blood glutamate levels was observed (P<0.001). Mild (33 to 36°C) and moderate (30 to 32°C) hypothermia led to reduced blood glutamate levels (P<0.001). Deep hypothermia (18 to 22°C) was associated with significant elevations in blood glutamate levels (P<0.001). Hypothermia, irrespective of the degree, led to elevations in GOT in plasma (P<0.001).

CONCLUSIONS

Mild and moderate hypothermia led to a reduction in blood glutamate levels in rats, whereas deep hypothermia was associated with a significant elevation in blood glutamate levels. We further demonstrated an elevation of GOT and GPT levels, supporting their involvement in reducing blood glutamate by the conversion of glutamate to 2-ketoglutarate. We suggest that the neuroprotective properties of hypothermia may be partially because of a blood glutamate-scavenging mechanism.

Comparison of parameters for detection of splanchnic hypoxia in children undergoing cardiopulmonary bypass with pulsatile versus nonpulsatile normothermia or hypothermia during congenital heart surgeries

The aim of this study is to evaluate gastric mucosal oxygenation together with whole-body oxygen changes in infants undergoing congenital heart surgery with cardiopulmonary bypass (CPB) procedure and the use of either pulsatile or nonpulsatile mode of perfusion with normothermia and pulsatile or nonpulsatile moderate hypothermia. Sixty infants undergoing congenital cardiac surgery were randomized into four groups as: nonpulsatile normothermia CPB (NNCPB, n = 15), pulsatile normothermia CPB (PNCPB, n = 15), nonpulsatile moderate hypothermia CPB (NHCPB, n = 15), and pulsatile moderate hypothermia CPB (PHCPB, n = 15) groups. In NNCPB and PNCPB groups, mild hypothermia was used (35°C), whereas in NHCPB and PHCPB groups, moderate hypothermia (28°C) was used. Gastric intramucosal pH (pHi), whole-body oxygen delivery (DO(2)) and consumption (VO(2)), and whole-body oxygen extraction fraction were measured at sequential time points intraoperatively and up to 2 h postoperatively. The measurement of continuous tonometry data was collected at desired intervals. The values of DO(2), VO(2), and whole-body oxygen extraction fraction were not different between groups before CPB and during CPB, whereas the PNCPB group showed higher values of DO(2), VO(2), and whole-body oxygen extraction fraction compared to the other groups at the measurement levels of 20 and 60 min after aortic cross clamp, end of CPB, and 2 h after CPB (P < 0.0001). Between groups, no difference was observed for pHi, lactate, and cardiac index values (P > 0.05). This study shows that the use of normothermic pulsatile perfusion (35°C) provides better gastric mucosal oxygenation as compared to other perfusion strategies in neonates and infants undergoing congenital heart surgery with CPB procedures.

Randomized comparison between mild and moderate hypothermic cardiopulmonary bypass for neonatal arterial switch operation

OBJECTIVES

To compare neonates receiving arterial switch operation (ASO) either with mild or moderate hypothermic cardiopulmonary bypass.

METHODS

Forty neonates undergoing ASO were randomized to receive either mild (Mi > 32 °C, n = 20) or moderate (Mo > 26 °C, n = 20) hypothermic cardiopulmonary bypass (CPB) between April 2007 and June 2010. All patients were diagnosed with simple transposition of the great arteries. Mean age (Mi: 8.32 ± 4.5 days, Mo: 7.54 ± 5.0 days, P = 0.21) and body weight were similar in both groups (Mi: 3.64 ± 0.91 kg, Mo: 3.73 ± 0.84 kg, P = 0.14). Follow-up was 3.1 ± 2.5 years for all patients.

RESULTS

Lowest perioperative rectal temperature was 33.5 ± 1.4 °C (Mi) versus 28.2 ± 2.1 °C (Mo) (P < 0.001). All patients safely weaned from CPB required lower doses of dopamine (Mi: 5.1 ± 2.4 µg/kg min, Mo: 6.5 ± 2.1 µ/kg min, P = 0.04), dobutamine (Mi: 7.2 ± 2.5 µg/kg min, Mo: 8.6 ± 2.4 µ/kg min, P = 0.04) and adrenalin (Mi: 0.02 ± 0.02 µg/kg min, Mo: 0.05 ± 0.03 µ/kg min, P = 0.03) in mild hypothermia group. Intraoperative blood transfusion (Mi: 190 ± 58 ml, Mo: 230 ± 24 ml, P = 0.03) and postoperative lactate levels (Mi: 2.7 ± 0.9 mmol/l, Mo: 3.1 ± 2.2 mmol/l, P = 0.02) were lower under mild hypothermia. Secondary chest closure was performed in 30% (Mi) versus 35% (Mo) (P = 0.65). Duration of inotropic support (Mi: 7 (4-11) days, Mo: 11 (7-15) days, P = 0.03), time to extubation (Mi: 108 (88-128) h, Mo: 128 (102-210) h, P = 0.04), lengths of intensive care unit (ICU) stay (Mi: 9 (5-14) days, Mo: 12 (10-18) days, P = 0.04) and hospital stay (Mi: 19 (10-29) days, Mo: 23 (15-37) days, P = 0.04) were significantly shorter under mild hypothermia. Two-year freedom from reoperation was 100% for both the groups.

CONCLUSIONS

The ASO under mild hypothermia seemed to be beneficial for pulmonary recovery, need for inotropic support and length of ICU and hospital stay. No worse early- or intermediate-term effects of mild hypothermia were found.

Brain temperature: heat production, elimination and clinical relevance

Neurological insults are a leading cause of morbidity and mortality, both in adults and especially in children. Among possible therapeutic strategies to limit clinical cerebral damage and improve outcomes, hypothermia remains a promising and beneficial approach. However, its advantages are still debated after decades of use. Studies in adults have generated conflicting results, whereas in children recent data even suggest that hypothermia may be detrimental. Is it because brain temperature physiology is not well understood and/or not applied properly, that hypothermia fails to convince clinicians of its potential benefits? Or is it because hypothermia is not, as believed, the optimal strategy to improve outcome in patients affected with an acute neurological insult? This review article should help to explain the fundamental physiological principles of brain heat production, distribution and elimination under normal conditions and discuss why hypothermia cannot yet be recommended routinely in the management of children affected with various neurological insults.

Therapeutic hypothermia

Therapeutic hypothermia, introduced more than 5 decades ago, remains an important neuroprotective factor in the surgery for the correction of congenital heart disease, in particular when intraoperative circulatory arrest is required. Hypothermia decreases cerebral metabolism and energy consumption and reduces the extent of degenerative processes such as the excitotoxic cascade, apoptotic and necrotic cell death, microglial activation, oxidative stress, and inflammation. Neurological outcome has become the focus of several studies in the recent years, and deep hypothermic circulatory arrest durations of more than 40 minutes are associated with increased mid- and long-term disability. Physiologic cerebral flow-metabolism coupling seems to be preserved with moderate and mild hypothermia, but cerebral blood flow autoregulation is probably altered after deep hypothermic circulatory arrest, suggesting disordered cerebral metabolism and oxygen use. Although evidence from animal studies suggests potential benefit from very low temperatures, postoperative development of choreoathetosis has been found to correlate with the degree of intraoperative hypothermia, recommending the use of central temperatures greater than 15 degrees C in the clinical practice. Cooling times longer than 20 minutes are needed to obtain homogeneous brain cooling and effective neuroprotection. Finally, there is evidence that the sites of temperature monitoring used in the clinical practice may underestimate brain temperature after cardiopulmonary bypass, with the risk of postoperative hyperthermic brain damage.

Study of hypothermia on cultured neuronal networks using multi-electrode arrays

Efficient and safe use of hypothermia during various neuro-medical procedures requires sound understanding of low temperature effects on the neuronal network's activity. In this report, we introduce the use of cultivated dissociated neuronal networks on temperature controlled multi-electrode arrays (MEAs) as a simple methodology for studying the long-term effects of hypothermia. The networks exhibit spontaneous activity in the form of synchronized bursting events (SBEs), followed by long intervals of sporadic firing. Through the use of our correlation method, these SBEs can be clustered into sub-groups of similar spatio-temporal patterns. Application of hypothermia to the network resulted in a reduction in the SBE rate, the spike intensity and an increase in inter-neuronal correlations. Within 2h following the cessation of hypothermia, the cultured network returned to its initial spatio-temporal SBE structure. These results suggest that the network survived cold exposure and demonstrate the feasibility of long-term continuous neural network recording during hypothermic conditions.

Enhancement of glutamate uptake mediates the neuroprotection exerted by activating group II or III metabotropic glutamate receptors on astrocytes

We investigated whether the activation of astroglial group II and III metabotropic glutamate receptors (mGluRs) could exert neuroprotective effects and whether the neuroprotection was related to glutamate uptake. Our results showed that the activation of astroglial group II or III mGluRs exerted neuroprotection against 1-methyl-4-phenylpyridinium (MPP+) astroglial conditioned medium-induced neurotoxicity in midbrain neuron cultures. Furthermore, MPP+ decreased glutamate uptake of primary astrocytes and C6 glioma cells, which was recovered by activating group II or III mGluRs. Specific group II or III mGluRs antagonists completely abolished the neuroprotective effects and the enhancement of glutamate uptake of their respective agonists. Our results showed that the primary cultured rat astrocytes and C6 glioma cells expressed receptor proteins for group II mGluR2/3, group III mGluR4, mGluR6 and mGluR7. C6 glioma cells expressed mRNA for group II mGluR3, group III mGluR4, mGluR6, mGluR7 and mGluR8. In conclusion, we confirmed that the activation of astroglial mGluRs exerted neuroprotection, and demonstrated that the mechanism underlying this protective role was at least partially related to the enhancement of glutamate uptake.

[The use of moderate hypothermia in the treatment of patients with severe craniocerebral trauma]

Traumatic brain injury initiates several metabolic processes that can increase the primary injury. It is well established that in severe head injuries, posttraumatic secondary insults, such as brain hypoxia, hypotension or anemia, exacerbate neuronal injury and lead to a poorer outcome. Experimental and clinical evidence suggests that moderate hypothermia (32-34 degrees C), may limit some of these deleterious secondary metabolic responses. Recent laboratory studies and prospective controlled clinical trials of induced moderate hypothermia for relatively short periods (24-48 h) in patients with severe head injury, have demonstrated good intracranial pressure control and better outcome when compared with patients maintained in normothermia and given conventional treatment. Despite its proven clinical role in neuroprotection, hypothermia research has been inconstantly followed for various reasons. In this paper we review the mechanisms of neuroprotection in hypothermia, the different preclinical and clinical studies that favor its use as a neuroprotector in severe head injury or in patients in whom high intracranial pressure is refractory to first tier measures. The evidence that favors hypothermia is discussed. We also discuss the negative results of the still unpublished multicentre trial on prophylactic moderate hypothermia developed in the USA. The main problem with moderate hypothermia is the lack of a systematic methodology to induce and maintain it. Also, optimal duration of its use and the methodology and timing for rewarming have not been determined. Consequently, the results of different trials are difficult to analyze and compare. However, most evidence suggests that hypothermia provides remarkable protection against the adverse effects of neuronal damage that is exacerbated by secondary injury. Further prospective controlled trials with clearly defined methodology are needed before this method is implemented in daily clinical practice. The most important task for the years to come may be to focus on refining this procedure, defining the optimal time of cooling and rewarming and to optimize the methods of rapidly inducing and maintaining low temperature. It is also essential to define the most appropriate method and velocity of the rewarming phase, in which many successfully controlled patients deteriorate and die.

Metabolic changes after cardiac surgery

The metabolic changes that occur after cardiac surgery result from a complex interaction between the effects of surgery and extracorporeal circulation per se, the inflammatory response to surgical trauma and extracorporeal circulation, perioperative use of hypothermia, the cardiovascular and neuroendocrine responses characteristic to cardiac surgery, and the drugs and blood products used to support circulation during and after operation. These changes include among others increased oxygen consumption and energy expenditure and increased secretion of insulin, growth hormone, adrenocorticotrophic hormone, cortisol, epinephrine and norepinephrine. Other changes include decreased total-Trijodthyronine levels, hyperglycemia, hyperlactatemia, increased glutamate, aspartate and free fatty acid concentrations, hypokalemia, an increased production of inflammatory cytokines and increased consumption of complement and adhesion molecules. There is evidence that better control of metabolic abnormalities improves the patients' outcome.